The small-size power plant based on the ORC process was developed particularly for reclamation of the heat lost from different heat-producing processes or machines, which cannot be used as such by heat transfer means or the like, due to the temperature of the lost heat in question or the conditions of the environment. In a small-size power plant, waste energy is usually converted by means of a turbine and a generator to electricity which can be easily utilized for different purposes. If high efficiency of the small-size power plant is achieved, the plant can also be used for small-scale energy production using fuel such as wood chips burned for this purpose.
It can be shown thermodynamically that converting such energy is best performed by a Rankine or ORC process based on circulation of an organic medium. The organic medium has a relatively small vaporization heat as compared with, for example water, the drop of its specific enthalpy in the turbine is small and the mass flow rate in relation to the output is high, whereby it is possible to achieve a high turbine efficiency even at small output rates.
A hermetic or fully closed circuit process has the advantage that there are no leaks and the process is thus reliable and durable in operation. The utilization of high-speed technology, whereby the turbine is directly coupled with a generator rotating at the same speed and thus producing high-frequency current, has made it possible to further simplify the process in a way that e.g. a separate reduction gear required by conventional processes as well as shaft inlets are not needed.
A hermetic energy converter unit of this kind, operating on high-speed technology and based on the ORC process, is known from the publication FI-66234, according to which the bearing of the rotor of the high-speed machine is carried out by an organic circulating medium, wherein the circulating medium is in a gaseous state. A previous patent application by the Applicant, FI-904720, discloses a method for securing the lubrication of the bearings in a hermetic high-speed machine.
The output of a single energy converter unit being used for applications in this connection is below 500 kW mainly because of constructional reasons. Naturally the total output of a small-size power plant may be significantly bigger by combining several energy converter units. The speed of rotation may vary considerably, in customary applications being generally over 8000 rpm, in power range from 200 kW to 400 kW most suitably between 18000 and 12000 rpm.
The process efficiency rates of small-size power plants are typically within the range of 10-21% depending on the size of the power plant, the circulating medium, the temperature of the incoming waste heat, and other similar factors, whereby the maximum efficiency that can normally be attained by an ORC process is 20-24%.
It is generally known that the efficiency of an aqueous steam process can be raised by reheating, because the average temperature of incoming heat is raised as explained for example in the Finnish publication Tekniikan Kasikirja II, p. 630. However, reheating is commonly used in relatively large power plants only, because two turbines operating at a different pressure level are needed. Similarly, a method is known from the publication mentioned above for reducing the superheating of the superheated aqueous steam by spraying water in it. Also this arrangement is utilized only in relatively large power plants.
The present invention make it possible to attain a significant improvement in plant efficiency as compared with that of a normal small-size power plant based on the ORC process. For achieving this aim, the method of the invention is mainly characterized in that, in the ORC the, ORC medium is intercooled by an intercooler, substantially in connection with the turbine and/or reheated in the vaporizer, wherein, first and second expansion phases in the turbine are carried out by the first and second turbine wheels of the turbine mounted on the rotor of the high-speed machine.
The most important advantages of the method of the invention are its simplicity and reliability of operation, whereby the method enables the application of a conventional technique, in connection with the ORC proces for improving the efficiency of a small-size power plant operating on high-speed technology.
Using the apparatus according to the invention, it is possible to utilize the ORC process in a simple and efficient manner in a small-size power plant giving a significantly better output than the present solutions. The efficiency is raised by means of the apparatus of the invention, whereby the net output of the small-size power plant is increased. Consequently, despite the capital investment in the additional arrangements required by the method, the total operating costs of the apparatus are significantly lower than with present solutions.